This invention relates to a microforce measurement method and an apparatus for measuring time-varying physical forces related to the movements of micro particles such as protein molecules.
Micro glass probes and optical tweezers are known as means for measuring pN-order microforces produced in the interactions of micro particles: for example, between a motor protein molecule and protein filaments.
When a force is applied to the end of a micro glass probe perpendicular to its axis, the probe bends and exhibits a displacement of which the magnitude is proportional to the applied force. In another case, when a force is applied to a particle caught by optical tweezers, the particle exhibits a displacement of which the magnitude is proportional to the applied force. Micro glass probes and optical tweezers are thereby utilized as micro spring scales.
With a motor protein fixed on the particle caught at the end of a glass probe or optical tweezers and brought close to protein filaments fixed on the observation plane, the particle caught by the glass probe end or optical tweezers is displaced when an interaction force is exerted between the motor protein molecule and the protein filaments. Measuring this displacement makes it possible to determine the time-varying force acting between the motor protein molecule and the protein filaments (for examples, TRENDS in Biotechnology, Vol.19, No.6, June 2001, p.211-216; and Japanese Patent Application No. 09-43434).
An intermolecular force microscope has also been disclosed, for example, in Japanese Patent Application No. 07-12825. This microscope performs non-contact measurement of attractive and repulsive forces perpendicular to the observation plane, maintaining constant distance between the observation plane and the probe through feedback control of the probe position normal to the observation plane using irradiation pressure provided by a laser.
However, these conventional techniques cannot control the distance between micro particles in parallel direction to the observation plane on the order of nm, because they are not designed to reduce background noise in this plane by eliminating thermal fluctuations in the probe and particles.
Particles such as protein molecules and the like fixed on the probe move in an axis perpendicular to the probe. Because the measurement laser is emitted along a predetermined direction to provide photon pressure, if the particle moves along the same axis as laser irradiation, its movement cannot be detected.
It is, therefore, an object of the present invention to provide a microforce measurement method and an apparatus capable of measuring 0.1 pN-order forces by reducing probe fluctuations to the order of a few angstroms and controlling on the order of a few nm the distance between a micro particle fixed on the probe, such as a cell or a molecule, and the micro particles fixed on the observation plane, such as cells or molecules.
The microforce measurement method according to the present invention for measuring microforces acting between a micro particle fixed on a probe and an observation plane involves the following steps: keeping the probe position motionless by applying irradiation pressure to the probe with a photon pressure laser through feedback control of probe position changes caused by movement of the particle fixed on the probe; and measuring and recording the laser output power varying with time during said feedback control.
The apparatus for the above method for measuring microforces acting between a micro particle fixed on a probe and an observation plane comprises: a photon pressure laser applying irradiation pressure to a probe; a feedback control means for keeping the probe position motionless by applying irradiation pressure to the probe with the photon pressure laser through feedback control of probe position changes caused by movement of the particle fixed on the probe; and a laser output power recording means for measuring and recording the laser output power varying with time during said feedback control.
The probe may have an almost plate-like shape and be positioned in the axis perpendicular to the observation plane. An objective lens may be installed before the probe in the light path of the photon pressure laser. The apparatus may further comprise an interferometric image generation means for projecting an interferometric image on a split photodiode, a probe position detection means for detecting the probe position based on the interferometric image, and a laser intensity control means for controlling, based on said detection result, the intensity of the photon pressure laser irradiated onto the probe. The feedback control means may hold the root-mean-square of probe fluctuations at approximately 1.2 nm or less. The probe may have a predetermined spring constant and further comprise an offset function of keeping the probe in a state of dynamic balance.
This configuration allows measurement of the particle moving along the same axis as laser irradiation. The offset force may be determined as desired. However, in the case of measurement of protein movement, the offset force should be in the 8-25 pN range, sufficiently greater than the force resulting from the protein movement.